[Relay][VM] Fix loading late bound consts when none exist (apache#10087)

* Fix loading late bound consts when none exist

* Simplify comment

* -mFix skipping of verilator tests

* Skip loading late-bound consts if none present

* Remove semi-related fix to verilator test skipping

* Remove more test-skip fixing for pr hygiene

* No-op for ci

* No-op for ci

src/runtime/vm/executable.cc

@@ -341,6 +341,10 @@ void Executable::MoveLateBoundConstantsToFile(const std::string& path, size_t by
 }
 
 void Executable::LoadLateBoundConstantsFromStream(dmlc::Stream* stream) {
+  if (late_bound_constant_names.empty()) {
+    VLOG(1) << "Found no late-bound constants to load";
+    return;
+  }
   ICHECK_EQ(late_bound_constant_names.size(), constants.size());
   Map<String, NDArray> map = runtime::LoadParams(stream);
   VLOG(1) << "loaded " << map.size() << " late-bound constants";

tests/python/relay/test_vm.py

@@ -1174,6 +1174,68 @@ def test_large_constants():
     tvm.testing.assert_allclose(expected, actual.numpy())
 
 
+def test_load_late_bound_consts_with_no_late_bound_consts():
+    """Check that load_late_bound_consts handles a model with no late bound consts."""
+    target = tvm.target.Target("llvm")
+    dev = tvm.cpu()
+
+    const_data = np.random.rand(1).astype("float64")
+    x = relay.var("x", shape=(1,), dtype="float64")
+    const = relay.const(const_data, dtype="float64")
+
+    func = relay.Function([x], relay.op.add(x, const))
+    mod = tvm.IRModule.from_expr(func)
+
+    vm_exec = vm.compile(mod, target=target)
+
+    temp = utils.tempdir()
+    path_consts = temp.relpath("consts")
+    path_dso = temp.relpath("lib.so")
+
+    # Ensure const_data is below the byte threshold for a late-bound const.
+    byte_limit = len(const_data.tobytes()) + 1
+    vm_exec.move_late_bound_consts(path_consts, byte_limit=byte_limit)
+    vm_exec.mod.export_library(path_dso)
+
+    mod = runtime.load_module(path_dso)
+    mod["load_late_bound_consts"](path_consts)
+
+    x_data = np.random.rand(1).astype("float64")
+    loaded_vm = runtime.vm.VirtualMachine(mod, dev)
+    actual = loaded_vm.invoke("main", x_data)
+    expected = x_data + const_data
+    tvm.testing.assert_allclose(expected, actual.numpy())
+
+
+def test_vm_save_and_load_without_designating_late_bound_consts():
+    """Check that a VM can be saved and loaded without late-bound consts in play.
+
+    Specifically, this test ensures that the machinery behind late-bound const
+    loading does not assume the need to load late-bound consts (and cause an error)
+    when the user did not choose to designate any consts as such.
+    """
+    target = tvm.target.Target("llvm")
+    dev = tvm.cpu()
+
+    const_data = np.random.rand(1).astype("float64")
+    x = relay.var("x", shape=(1,), dtype="float64")
+    const = relay.const(const_data, dtype="float64")
+
+    func = relay.Function([x], relay.op.add(x, const))
+    mod = tvm.IRModule.from_expr(func)
+
+    vm_exec = vm.compile(mod, target=target)
+
+    code, lib = vm_exec.save()
+    exe = runtime.vm.Executable.load_exec(code, lib)
+
+    x_data = np.random.rand(1).astype("float64")
+    loaded_vm = runtime.vm.VirtualMachine(exe, dev)
+    actual = loaded_vm.invoke("main", x_data)
+    expected = x_data + const_data
+    tvm.testing.assert_allclose(expected, actual.numpy())
+
+
 if __name__ == "__main__":
     import sys